This invention relates to a method of preparing improved sorbents for the removal of sulfur compounds from sulfur-bearing gases.
Air pollution with sulfur dioxide is a major problem in the United States today. Sulfur dioxide is objectionable principally because above relatively low concentrations it is toxic to human beings and animals and is destructive to vegetation. Sulfur dioxide and its oxidation products, sulfur trioxide and sulfuric acid, are a major source of acidity in rain and fog which in turn can be corrosive.
Industrial plants that utilize sulfur-bearing commercial fuels such as coal or residual oil in the production of glass, lime, cement, ceramics, metals, and/or electric power, etc., are major sources of sulfur dioxide emissions to the atmosphere. Moreover, plants that utilize these raw fuels as feed materials in the production of refined fuels or chemicals such as coke, ammonia, methanol, formaldehyde, methane, and industrial gases also involve the production of reducing gases containing hydrogen, carbon monoxide and hydrogen sulfide.
Objectionable sulfur dioxide-bearing gases also are generated as waste gases in the smelting of sulfur-bearing minerals, the refining of sulfur-containing crude oils, the synthesis of sulfuric acid, the sulfonation of hydrocarbons, the production of sulfur by the Claus process, the production of paper by way of a wood-pulping process, and similar industrial processes.
Furthermore, the discharge of gas streams containing sulfur dioxide into the atmosphere constitutes a waste of a useful material because the sulfur content thereof is a potentially useful industrial commodity. Currently, tens of millions of tons of sulfur oxides are released into the atmosphere over populated regions of the United States each year. Thus, the recovery of some of this sulfur either as such or in another form could result in the accumulation of a supply of useful chemicals of significant value.
Many processes are available and/or have been proposed both for the removal of sulfur dioxide from oxygen bearing combustion gases and/or for the removal of hydrogen sulfide from reducing gases. Most of these processes involve scrubbing the gases in contact with organic or aqueous solutions of alkaline chemicals to extract the acid sulfur compounds by either physical and/or chemical reactions between components of the gases and liquids. The reactive chemicals utilized in these processes include ammonia, organic amines, and oxides, carbonates and sulfites of alkali and alkaline earth metals.
Wet processes of the type discussed above, however, generate sulfur-bearing solutions and/or slurries that must be either regenerated or discarded as waste. Moreover, the combined treatment and regeneration processes are expensive in both capital and operating costs, and the sulfur-bearing effluents, when discarded, frequently generate alternate water pollution problems.
The scrubbing of gases in contact with solutions also involves cooling the gases to temperatures near to or below the boiling point of the scrubbing liquor. Wet processes, therefore, have the disadvantage of both operating at low temperatures and of generating product gases that are saturated with water vapor. Moreover, if these cold gases are released to the atmosphere they will remain near ground level, and frequently will pollute the local ambient air more seriously than would the untreated but hot flue gas.
The wet processes are particularly disadvantageous when utilized in the treatment of hot gases. Moreover, when utilized in the desulfurization of gases such as the hot reducing gases from the partial combustion of sulfur-bearing coal or fuel oil, etc., these processes are unusually costly in heat transfer equipment and wasteful in energy consumption.
Other processes that have been either utilized or proposed for use in the desulfurization of either combustion gases containing sulfur and oxygen, and/or reducing gases containing hydrogen sulfide, involve contacting the gases with dry solid sorbents. Processes of this type may be sub-divided into general classifications as follows:
a. Processes that utilize the physical properties of sorbents such as activated alumina, activated carbon, and silica gel, and operate at low temperatures; and PA1 b. Processes that utilize minerals and chemicals such as dolomite, magnesite, calcite, siderite, magnetite, hematite, bauxite, lime, soda ash and/or magnesia, etc., as solid sorbents containing components that react chemically with sulfur compounds in the gas, and operate at relatively high temperatures.
Processes of the type described in (a) above, which operate at low temperatures, have disadvantages similar to those of the wet processes described previously. Moreover, sorbents that utilize the physical properties of solids to separate components of gases, are generally low in capacity, low in operating life, and costly both to produce and to regenerate.
Processes of the type described in (b) above, which utilize dry minerals and/or chemicals as active sorbents for sulfur from gases, have been proposed for use, and some have been used commercially in the desulfurization of both combustion gases containing oxygen and oxides of sulfur, and reducing gases containing carbon monoxide, hydrogen, and hydrogen sulfide. Some of these processes have involved addition of the sorbents to the combustion fuel, and discard of the reacted products with the ash or residue from the combustion furnace. Other similar processes have involved partial oxidation of the fuel with air or oxygen and steam, desulfurization of the reducing gas products in contact with the solid sorbents, and discard of the spent sorbent as solid waste.
The minerals or chemicals utilized in the above type of processes, such as limestone, lime, dolomite or soda ash, etc., generally are employed at a ratio higher than stoichiometric to sulfur in the fuel, and the finely divided of the reacted solids are difficult to separate from the gas. Moreover, the large quantities of sorbent used in these processes are costly to obtain, and the spent sorbents are costly to discard as solid waste, and after discard are a potential source of water pollution.
When commercial fuels such as coal or residual oil are utilized as raw materials in the production of ammonia, methanol, methane, fuel gas, or combined cycle electric power, etc., an early step of the conversion process generally involves partial combustion of these fuels with air or oxygen and steam, to produce hot reducing gases containing hydrogen and carbon monoxide. Moreover, when the available fuels contain sulfur, the hot reducing gases are contaminated with hydrogen sulfide and carbonyl sulfide, etc., and generally must be desulfurized for use in the synthesis or conversion processes.
Conventional processes for the desulfurization of hot reducing gases of the above type, generally involve cooling the hot gas, desulfurizing the cooled gas in contact with aqueous solutions of alkaline chemicals, and reheating the desulfurized gas for use in the conversion processes. Processes of this type generally utilize heat exchange equipment of alloy construction, and generally are costly in capital investment and wasteful in energy consumption.
Alternative processes for the desulfurization of reducing gases in contact with minerals or chemicals at high temperatures have been proposed, and some of these processes have been test operated in pilot plant equipment. Most of these processes involve the use of limestone or dolomite as a sorbent for sulfur, and involve chemical reactions between calcium in the minerals and sulfur in the gas to produce calcium sulfide and/or calcium sulfate in the reacted solids.
Processes of the above type, however, utilize calcium at a high ratio to sulfur on a stoichiometric basis, consume large quantities of sorbents, and generate large quantities of solid waste. Moreover, both the calcium sulfide and sulfate components of the spent sorbent are both highly stable and unusually costly to regenerate for reuse as sorbents in the process.
In view of the increasing demand for energy, the decreasing supply of low sulfur fuels, and the large known reserves of sulfur-bearing coal, there is an increasing demand for a low cost process for the desulfurization of sulfur-bearing gases.